1. Field of the Invention
This invention relates generally to equipment for use in the extraction of oil from oleaginous matter using an organic solvent.
2. Description of Related Art
The production of crude oil from oleaginous matter, such as but not limited to soybeans, rapeseed, sunflower seed, peanuts, cottonseed, palm kernels, and corn germ, starts with the mechanical and thermal preparation of the oleaginous matter to remove external coverings and expose the cellular inner structure containing the oil. If the oil content of the oleaginous matter is less than typically 30% by weight, the prepared oleaginous matter goes directly to the solvent extractor in the form of a flake or pellet. If the oil content of the prepared oleaginous matter is greater than 30% by weight, a portion of the oil contained in the oleaginous matter is removed via pressure in a mechanical screw press and the pressed oleaginous matter goes to the solvent extractor in the form of a press cake.
The prepared oleaginous material in the form of flakes, pellets or cake is conveyed from the seed preparation process to the solvent extraction process and enters the solvent extractor where it is treated with an organic solvent, such as but not limited to n-hexane and its isomers, to solvent extract the oil.
The solvent extractor conveys the prepared material from its inlet to its exit, providing the prepared material approximately 30 minutes to 120 minutes of residence time. While the material is being conveyed forward, miscella (the solution of oil in the extraction solvent) is washed down through the layer of material to wash out the vegetable oil. Each successive miscella wash is of a decreasing concentration of vegetable oil. After four to twelve miscella washes, the material is washed once more by fresh solvent, ending the extraction process. Before the material leaves the extractor, it is allowed to gravity drain to reduce the amount of retained solvent. The extracted, spent material then falls into the extractor discharge and is made to leave the apparatus. The miscella with the highest concentration of vegetable oil also exits the apparatus to a full-strength miscella tank.
For the solvent extraction of prepared oleaginous material, two main types of extractors are commonly used. There is the so-called immersion extractor employing a deep bed of material that is typically greater than 2 m high, and there is the percolation extractor employing a shallow bed of material that is typically less than 1 m high. In both extractor types, the solvent and miscella pass through the material layer in a vertically downward manner in multiple countercurrent passes. In the immersion extractor, the solvent and miscella flow rates are sufficiently high to displace all vapor between the particles of material resulting in miscella or solvent pooling above the layer, and in the percolation extractor the solvent and miscella flow rates are sufficiently low to cascade the solvent and miscella through the material without displacing all vapor between the particles of material.
An early embodiment of the immersion extractor has been disclosed in U.S. Pat. No. 2,790,708. These early embodiments had capacities up to 250 tons per day but subsequently, much larger immersion extractors have been developed as disclosed for instance in U.S. Pat. No. 3,860,395. According to an article by one of the inventors (K. Weber, J. Am. Oil Chem. Soc., volume 58, pages 538-539, 1981), its daily capacity had been increased to 3000 tons.
Such immersion extractors have been found to operate satisfactorily with soybean flakes but when the material to be extracted contains more fines, the rate at which the solvent passes downwards through the bed of material is reduced significantly. Because a certain number of passes is required to attain a baseline extraction efficiency, these fines lower the extractor capacity. In that case a more shallow bed is preferred. This has led to the development of a type of extractor according to U.S. Pat. No. 2,907,640 which discloses an apparatus comprising a revolvably mounted substantially liquid-permeable endless belt having a substantially horizontally extending portion, a separate chain of upright, open bottomed, box-like frames movable along with the horizontally extending portion of the belt subdividing the same into framed flexibly linked segments, means for passing a solid material on to one end of the horizontally extending portion of the belt for movement therealong, means for removing solid material coming off the other end of the horizontally extending portion of the belt, means for passing solvent in contact with material passing along the horizontally extended portion of the belt, means for collecting at least a portion of the solvent after the contacting and means for recycling the same in contact with material passing along the horizontally extending portion of the belt, means for the belt and means for revolving the chain of frames at substantially the same velocity as the endless belt. U.S. Pat. No. 2,907,640 also discloses an extractor comprising a second revolvably mounted substantially liquid permeable endless belt onto which material falls from the first belt and travels therealong. Since this second belt is located within the same housing, it increases the extraction capacity of the apparatus.
Since the publication of U.S. Pat. No. 2,907,640, this type of shallow bed, percolation extractor has been much improved. It is extremely heavy, upright, open bottomed, box-like frames movable along with the horizontally extending portion of the belt subdividing the same into framed flexibly linked segments have been replaced by a much lighter assembly of chains kept apart by baffles that also ensure the forward movement of the material being extracted. This much lighter construction reduces power requirement and investment cost and also eliminates the channeling of solvent along internal vertical surfaces. A description of such a continuous loop extractor can be found in the Practical Handbook of Soybean Processing and Utilization (Editor D. R. Erickson, AOCS Press, Urbana, Ill. and United Soybean Board, St Louis Mo., pages 82 and 84, 1995).
Solvent extraction equipment used in processing plants is commonly judged by its mechanical reliability and related safety of operation and by the residual oil content (ROC) of the ensuing meal because lowering the ROC has great financial consequences. An extractor with a daily capacity of 3000 tons will process about 1 million tons per annum and, in the case of soybeans, produce some 0.75 million tons of meal. A reduction of 0.1% by weight in ROC then corresponds to 750 tons of oil per annum, and since the selling price difference between crude soybean oil and the meal can reach US $500 per ton, even this small reduction leads to a net saving of at least US $375,000 per annum.
There are six parameters that affect the ROC performance of the solvent extractor apparatus. These six parameters are: contact time, particle thickness, extractor temperature, miscella flow rate, number of miscella stages and solvent retention.
Contact time. The total time that the oleaginous material spends in the extractor is the residence time. Residence time can be subdivided into wash time and drain time. Wash time is the time the oleaginous material spends under the washing nozzles of the extractor, and drain time is the time the oleaginous material spends draining prior to discharge. Wash time can be further subdivided into contact time and dormant time. Contact time is the time a particle of oleaginous material spends in the washing zone of the extractor where the particle is in contact with miscella. Extraction only takes place during contact time. Adequate contact time is critical for maximizing extraction efficiency and minimizing the amount of residual oil remaining in the oleaginous material.
Particle thickness. Various oleaginous materials are prepared for extraction using different process steps, but with virtually all oleaginous materials, one process step is flaking. The principle purpose of flaking is to reduce the thickness of the oleaginous material and thereby reduce the distance and number of cell walls that miscella needs to diffuse through to reach the oil bodies. By reducing particle thickness, desired ROC results can be achieved with less contact time.
Extractor Temperature. As the temperature of the miscella increases, its rate of diffusivity through the cell walls of the oleaginous material increases. Since the prepared oleaginous material enters the extractor at approximately 60° C., and both the oil and meal fractions are heated to above 100° C. in subsequent process steps, there is no extra energy required for operating the extractor at a warm temperature. As a result, optimizing extraction results requires operating the extractor as warm as safely possible.
Miscella Flow Rate. The miscella flow rate is the maximum volumetric flow rate of miscella that can flow down through the bed of material per unit of material bed surface area. The miscella flow rate is determined by the screen below the bed of material. The material bed is approximately 40 to 50 percent solids and 50 to 60 percent void space. Therefore, as the miscella is moving downward, it has 50 to 60 percent open area. The screen under the material bed has less open area, and therefore the material interface with the screen creates the greatest restriction to flow. As downward miscella flow reaches the miscella flow rate, the material/screen interface reaches its maximum flow rate and begins restricting the flow of miscella. All void spaces between the oleaginous material particles fill with miscella as the solvent vapors are pushed out the top of the material bed. At each washing stage of the extractor, miscella needs to have an opportunity to wash the material bed, pass through the screen, and then enter the proper miscella collection receptacle underneath the material bed. For a given extractor and prepared oleaginous material, each miscella collection receptacle is carefully calculated to be located a specific distance after its washing nozzle to maintain separation between washing stages.
Number of Miscella Stages. In most extraction applications, the prepared material contains approximately 20% oil by weight and the goal is to reduce the oil content to approximately half of one percent by weight. The minimum number of miscella stages can be calculated for a given solvent to material ratio. For an energy competitive distillation system, the solvent to material ratio should be maintained below one. To achieve a ratio of one or less, the minimum number of stages required can be calculated to be four stages. A four-stage extractor is sufficient in a theoretical sense, but leaves no contingency for lack of achieving equilibrium at each miscella stage. The more miscella stages, the greater the theoretical extraction efficiency will be. In practice, however, if an extractor is designed with too many miscella stages, this causes the individual stages to have insufficient contact time to reach equilibrium so that adding further stages will not reduce ROC. Commercially, most extractors have in the range of five to ten miscella stages.
Solvent Retention. After the washing zone of the extractor, the extracted material is left to gravity drain. This gravity drain time is generally in the range of five to twenty minutes. After gravity drainage, the solvent retained by the extracted material will generally be in the range of 25 to 35 percent. The solvent retained by the flakes is actually weak miscella. This miscella typically contains approximately half of one percent oil. In the meal desolventizer, the solvent is evaporated, leaving behind the traces of oil, often referred to as the residual oil. In order to minimize the residual oil left in meal, it is important to minimize the amount of weak miscella carried forward to the meal desolventizer.